In order to configure a wavelength-division-multiplexed passive optical network (WDM-PON) using a conventional Reflective Semiconductor Optical Amplifier (RSOA), there has been proposed a method in which an optical signal that was modulated into downstream data by a central office (CO) is transmitted to a RSOA of an optical network terminal (ONT) and the RSOA is operated at a gain saturation region with respect to the power of the input optical signal to greatly reduce a difference between level ‘0’ and level ‘1’ of the input optical signal when the input optical signal is remodulated to upstream data.
In such an optical signal reusing method, when the gain saturation of the RSOA occurs at lower optical power, an optical power budget in an optical link increases, and therefore, gain saturation power needs to be reduced to be as little as possible and an optical signal input to the RSOA needs to be amplified sufficiently for upstream transmission. Consequently, a gain of the RSOA needs to be large enough.
However, in actuality, the compressing ability of the RSOA in the gain saturation region is limited, and therefore, there is a limit in reducing an extinction ratio (ER) of the input optical sufficiently. In this case, when the residual extinction ratio is directly modulated by the upstream data again, the residual extinction ratio is reflected to the thickness of the level ‘1’. As the level ‘1’ is thicker, upstream transmission quality is getting worse. If the level ‘1’ is thicker than a certain thickness, the upstream transmission quality is getting worse rapidly.
Accordingly, the extinction ratio of a downstream optical signal may need to be decreased to a minimum, which is just enough for transmission. At this time, due to the low extinction ratio of the downstream optical signal, power penalty may occur in downstream transmission. Especially, when the optical wavelengths of apparatuses configuring a downstream link are even slightly misarranged, there appears sensitivity by which the extinction ratio of an optical signal input into a receiver is getting less than the extinction of an optical signal output from a transmitter, and the transmitting quality is getting worse rapidly.
Recently, against activation of sound/data/broadcast hybrid services, research and development for a Wavelength Division Multiplexing (WDM)-based network are actively progressed throughout the world.
Communications between a central base station and a terminal in a WDM-PON are performed using wavelengths which are designated in each terminal. In the WDM-PON, since a private wavelength is used for each terminal, excellent security is guaranteed, a large amount of communication services are possible, and different transmission techniques (for example, different data rates, different frame formats, etc.) can be applied to respective terminals or services.
A downstream optical wavelength reusing method, among a variety of WDM-PON configuration methods that have been proposed to date, generates a upstream optical signal by reusing a downstream optical wavelength. Thus, a upstream optical wavelength is equal to a downstream optical wavelength, a Free Spectral Range (FSR) of a WDM-based multiplexing/demultiplexing unit does not need to be considered, also a multi-stage remote node can be configured, and a variety of subscriber network types can be configured.
Hereinafter, a WDM-PON using the downstream optical signal reusing method will be described in detail.
FIG. 13 illustrates the configuration of a WDM-PON according to a conventional downstream optical signal reusing method. Referring to FIG. 13, the WDM-PON includes an Optical Line Terminal (OLT) 1300, an optical line 1310, a wavelength multiplexing/demultiplexing unit 1320, and Optical Network Units (ONUs) 1330.
First, the respective elements of the OLT 1300 will be described below, wherein the OLT 130 is located in a telephone office.
The OLT 1300, as illustrated in FIG. 13, includes a protocol processing unit 1301, a plurality of wavelength fixed optical transmitters Tx#1 through Tx#N 1302, a wavelength multiplexer 1303, an optical circulator 1304, a wavelength demultiplexer 1305, and a plurality of optical receivers 1306.
If a downstream electrical signal is transferred from the protocol processing unit 1301 to the wavelength fixed optical transmitters Tx#1 through Tx#N 1302, each wavelength fixed optical transmitter 1302 outputs an optical signal corresponding to the downstream electrical signal to the wavelength multiplexer 1303.
Then, the wavelength multiplexer 1303 combines optical signals received from the wavelength fixed optical transmitters Tx#1 through Tx#N 1302 with each other, and then transfers the resultant signal via the optical line 1310 to the wavelength multiplexing/demultiplexing unit 1320 which is located at a remote node. The optical signals are divided according to their wavelengths by the wavelength multiplexing/demultiplexing unit 1320. The wavelength multiplexing/demultiplexing unit 1320 operates as a wavelength demultiplexer when a downstream signal is received, and operates as a wave multiplexer when a upstream signal is received.
The optical signals which have been divided according to their wavelengths by the wavelength multiplexing/demultiplexing unit 1320 are transferred to the corresponding ONU 1330.
Hereinafter, the elements of each ONU 1330 will be described in detail. Here, each ONU 1330 includes a tap coupler 1331, an optical transmitter 1332, an optical receiver 1333, and a protocol processing unit 1334. Here, the optical transmitter 1332 may be a wavelength-agnostic semiconductor optical amplifier.
The tap combiner 1331 transfers some of downstream optical signals transmitted from the wavelength demultiplexer 1320 to the optical receiver 1333 to restore them as downstream signals, and transfers the remaining part of the downstream optical signals to the optical transmitter 1332.
Then, if the optical transmitter 1332 receives an upstream electrical signal from the protocol processing unit 1334, the optical transmitter 1332 generates a upstream optical signal by reusing the downstream optical signal received from the tap combiner 1331 and transmits the upstream optical signal to the ONU 1330.
Since the optical transmitter 1332 uses a semiconductor optical amplifier which is a wavelength agnostic light source, no inventory problem occurs and an optical transmitter type can process all WDM channels.
However, in the above-described WDM-PON using the conventional downstream optical signal reusing method, since an optical transmitter which is used in a telephone office on the WDM-PON is a wavelength fixed type optical transmitter (for example, a DFB-LD which outputs the same optical wavelength, a wavelength fixed external resonance laser, a Vertical Cavity Surface Emitting Laser (VCSEL), etc.) using a wavelength fixed light source, different types of optical transmitters corresponding to the number of wavelengths that are available in the telephone office are needed. The manufacturing, installation and management of light sources according to wavelengths give a great load to all of users and providers, and thus increase system costs. That is, in the WDM-PON according to the conventional downstream optical signal reusing method, an inventory problem occurs that different types of optical transmitters should be provided for the operation, management, replacement, etc. of systems.